The present invention relates to a direct spread spectrum signal receiving apparatus and a synchronism acquisition circuit for use in the receiving apparatus.
As is well-known, a direct spread spectrum signal receiving apparatus includes a synchronism acquisition circuit for detecting both an amount of phase correction necessary for despread and an amount of correction of frequency deviation of carrier wave in response to a received spread spectrum signal.
A prior art synchronism acquisition circuit will now be described with reference to the accompanying drawings. FIG. 7 is a block diagram of the arrangement of the synchronism acquisition circuit.
A spread spectrum signal is received by a reception unit of a receiving apparatus (not shown) and then converted to a digital signal by an A/D converter. This digital signal is input to an FFT circuit 71 as spread spectrum signal data y(nTs), where n is an arbitrary constant and Ts is a sampling period of the A/D converter.
In the FFT circuit 71, the spread spectrum signal data y(nTs) undergoes the fast Fourier transform at an N (natural number) point to be transformed from a signal in a time domain to a spread spectrum signal series Yk (k=0, 1, 2, . . . , N-1) in a frequency domain, and the spread spectrum signal series Yk is input to a correlation value detector 72. This signal series YK is given as follows. ##EQU1##
The correlation value detector 72 includes convolution arithmetic units (CONV) 721 to 72L (L=a constant at which Ts coincides with the period of a spread spectrum code). A spread spectrum code series Pk.sup.(i) (i=0, 1, 2, . . . , L-1) and spread spectrum signal series Yk are input to each of the convolution arithmetic units 721 to 72L.
The spread spectrum code series Pk.sup.(i) is a signal series in the frequency domain, into which a spread spectrum code signal p{(n-1)Ts} having a spread spectrum signal phase iTs is converted by the fast Fourier transform at an N point, using an FFT circuit (not shown). The following are equations expressing spread spectrum code series Pk.sup.(i) and Pk.sup.(0). ##EQU2##
The convolution arithmetic units 721 to 72L perform a convolution operation for spread spectrum code series Pk.sup.(i) and spread spectrum signal series Yk in the frequency domain to obtain a correlation value Xk.sup.(i) between them. In other words, the correlation value detector 72 detects a correlation value Xk.sup.(i) between spread spectrum code series Pk.sup.(i) and spread spectrum signal series Yk whose phases are varied Ts by Ts.
The results of L convolution operations (correlation values Xk.sup.(i)) so obtained, are illustrated in FIGS. 8A-8D and the equation thereof is given as follows: ##EQU3##
The correlation values Xk.sup.(i) are input to their corresponding square circuits 731 to 73L and squared therein, and the squared values are supplied to their corresponding adders (.SIGMA.) 741 to 74L.
Each of the adders 741 to 74L adds M correlation values Xk.sup.(i) the powers of which are converted by its corresponding one of the square circuits 731 to 73L to average the correlation values corresponding to spread spectrum signal data from y(nTs) to y{(n+MN)Ts}. The average value is then input to a maximum-value The average value is then input to a maximum-value selection circuit 75.
The maximum-value selection circuit 75 selects the maximum one of the average values of the correlation values obtained from the adders 741 to 74L. Based on the selected maximum average value, both an amount of phase correction of a spread spectrum signal and that of correction of frequency deviation of carrier wave are obtained.
In the case of FIGS. 8A-8D, the largest correlation value is obtained when i=2 as shown in FIG. 8C. The maximum-value selection circuit 75 thus finds an amount of phase correction of a spread spectrum signal from the phase of the signal when i=2, and obtains an amount of correction of frequency deviation of carrier wave from a difference df between frequency f2, which represents the largest correlation value when i=2, and reference frequency fs.
The amounts of phase correction and correction of frequency deviation of the spread spectrum signal, so obtained, are supplied to a frequency correction circuit and a despread arithmetic circuit (neither of which is shown) at the subsequent stage.
In the prior art synchronism acquisition circuit having the above-described constitution, a convolution operation is performed for the spread spectrum signal data and spread spectrum code signal in the frequency domain, so that the frequency domain necessitates the FFT circuit 71. Since, however, the FFT circuit 71 performs a complicated operation and its size is large, the synchronism acquisition circuit using this circuit will be complicated.